diff --git a/src/execute_request.jl b/src/execute_request.jl
index fe4c3fc3..88da0bc6 100644
--- a/src/execute_request.jl
+++ b/src/execute_request.jl
@@ -5,17 +5,49 @@
 import Base.Libc: flush_cstdio
 import Pkg
 
-const text_plain = MIME("text/plain")
-const image_svg = MIME("image/svg+xml")
-const image_png = MIME("image/png")
-const image_jpeg = MIME("image/jpeg")
-const text_markdown = MIME("text/markdown")
-const text_html = MIME("text/html")
-const text_latex = MIME("text/latex") # Jupyter expects this
-const text_latex2 = MIME("application/x-latex") # but this is more standard?
-const application_vnd_vega_v3 = MIME("application/vnd.vega.v3+json")
-const application_vnd_vegalite_v2 = MIME("application/vnd.vegalite.v2+json")
-const application_vnd_dataresource = MIME("application/vnd.dataresource+json")
+Base.showable(a::AbstractVector{<:MIME}, x) = any(m -> showable(m, x), a)
+
+"""
+A vector of MIME types (or vectors of MIME types) that IJulia will try to
+render. IJulia will try to render every MIME type specified in the first level
+of the vector. If a vector of MIME types is specified, IJulia will include only
+the first MIME type that is renderable (this allows for the expression of
+priority and exclusion of redundant data).
+
+For example, since "text/plain" is specified as a first-child of the array,
+IJulia will always try to include a "text/plain" representation of anything that
+is displayed. Since markdown and latex are specified within a sub-vector, IJulia
+will always try to render "text/markdown", and will only try to render
+"text/latex" if markdown isn't possible.
+"""
+const ijulia_mime_types = Vector{Union{MIME, AbstractVector{MIME}}}([
+    MIME("text/plain"),
+    MIME("image/svg+xml"),
+    [MIME("image/png"),MIME("image/jpeg")],
+    [
+        MIME("text/markdown"),
+        MIME("text/html"),
+        MIME("text/latex"), # Jupyter expects this
+        MIME("application/x-latex"), # but this is more standard?
+    ],
+])
+
+"""
+MIME types that when rendered (via stringmime) return JSON data. See
+`ijulia_mime_types` for a description of how MIME types are selected.
+
+This is necessary to embed the JSON as is in the displaydata bundle (rather than
+as stringify'd JSON).
+"""
+const ijulia_jsonmime_types = Vector{Union{MIME, Vector{MIME}}}([
+    [MIME("application/vnd.vegalite.v2+json"), MIME("application/vnd.vega.v3+json")],
+    MIME("application/vnd.dataresource+json"),
+])
+
+register_mime(x::Union{MIME, Vector{MIME}})= push!(ijulia_mime_types, x)
+register_mime(x::AbstractVector{<:MIME}) = push!(ijulia_mime_types, Vector{Mime}(x))
+register_jsonmime(x::Union{MIME, Vector{MIME}}) = push!(ijulia_jsonmime_types, x)
+register_jsonmime(x::AbstractVector{<:MIME}) = push!(ijulia_jsonmime_types, Vector{Mime}(x))
 
 include("magics.jl")
 
@@ -23,36 +55,63 @@ include("magics.jl")
 # in Jupyter display_data and pyout messages
 metadata(x) = Dict()
 
-# return a String=>String dictionary of mimetype=>data
-# for passing to Jupyter display_data and execute_result messages.
-function display_dict(x)
-    data = Dict{String,Any}("text/plain" => limitstringmime(text_plain, x))
-    if showable(application_vnd_vegalite_v2, x)
-        data[string(application_vnd_vegalite_v2)] = JSON.JSONText(limitstringmime(application_vnd_vegalite_v2, x))
-    elseif showable(application_vnd_vega_v3, x) # don't send vega if we have vega-lite
-        data[string(application_vnd_vega_v3)] = JSON.JSONText(limitstringmime(application_vnd_vega_v3, x))
-    end
-    if showable(application_vnd_dataresource, x)
-        data[string(application_vnd_dataresource)] = JSON.JSONText(limitstringmime(application_vnd_dataresource, x))
+"""
+Generate the preferred MIME representation of x.
+
+Returns a tuple with the selected MIME type and the representation of the data
+using that MIME type.
+"""
+function display_mimestring(mime_array::Vector{MIME}, x)
+    for m in mime_array
+        if showable(mime_array, x)
+            return display_mimestring(m, x)
+        end
     end
-    if showable(image_svg, x)
-        data[string(image_svg)] = limitstringmime(image_svg, x)
+    error("No displayable MIME types in mime array.")
+end
+
+display_mimestring(m::MIME, x) = (m, limitstringmime(m, x))
+
+"""
+Generate the preferred json-MIME representation of x.
+
+Returns a tuple with the selected MIME type and the representation of the data
+using that MIME type (as a `JSONText`).
+"""
+function display_mimejson(mime_array::Vector{MIME}, x)
+    for m in mime_array
+        if showable(mime_array, x)
+            return display_mimejson(m, x)
+        end
     end
-    if showable(image_png, x)
-        data[string(image_png)] = limitstringmime(image_png, x)
-    elseif showable(image_jpeg, x) # don't send jpeg if we have png
-        data[string(image_jpeg)] = limitstringmime(image_jpeg, x)
+    error("No displayable MIME types in mime array.")
+end
+
+display_mimejson(m::MIME, x) = (m, JSON.JSONText(limitstringmime(m, x)))
+
+"""
+Generate a dictionary of `mime_type => data` pairs for all registered MIME
+types. This is the format that Jupyter expects in display_data and
+execute_result messages.
+"""
+function display_dict(x)
+    data = Dict{String, Union{String, JSONText}}()
+    for m in ijulia_mime_types
+        if showable(m, x)
+            mime, mime_repr = display_mimestring(m, x)
+            data[string(mime)] = mime_repr
+        end
     end
-    if showable(text_markdown, x)
-        data[string(text_markdown)] = limitstringmime(text_markdown, x)
-    elseif showable(text_html, x)
-        data[string(text_html)] = limitstringmime(text_html, x)
-    elseif showable(text_latex, x)
-        data[string(text_latex)] = limitstringmime(text_latex, x)
-    elseif showable(text_latex2, x)
-        data[string(text_latex)] = limitstringmime(text_latex2, x)
+
+    for m in ijulia_jsonmime_types
+        if showable(m, x)
+            mime, mime_repr = display_mimejson(m, x)
+            data[string(mime)] = mime_repr
+        end
     end
+
     return data
+
 end
 
 # queue of objects to display at end of cell execution
diff --git a/test/execute_request.jl b/test/execute_request.jl
index 52e125fe..3ac73b09 100644
--- a/test/execute_request.jl
+++ b/test/execute_request.jl
@@ -1,5 +1,7 @@
 using Test
+using Base64, JSON
 
+import IJulia
 import IJulia: helpmode, error_content, docdict
 
 content = error_content(UndefVarError(:a))
@@ -7,3 +9,49 @@ content = error_content(UndefVarError(:a))
 
 @test haskey(docdict("import"), "text/plain")
 @test haskey(docdict("sum"), "text/plain")
+
+struct FriendlyData
+    name::AbstractString
+end
+
+@testset "Custom MIME types" begin
+    friend = FriendlyData("world")
+
+    FRIENDLY_MIME_TYPE = MIME"application/vnd.ijulia.friendly-text"
+    FRIENDLY_MIME = FRIENDLY_MIME_TYPE()
+    Base.Multimedia.istextmime(::FRIENDLY_MIME_TYPE) = true
+    Base.show(io, ::FRIENDLY_MIME_TYPE, x::FriendlyData) = write(io, "Hello, $(x.name)!")
+    IJulia.register_mime(FRIENDLY_MIME)
+
+    BINARY_MIME_TYPE = MIME"application/vnd.ijulia.friendly-binary"
+    BINARY_MIME = BINARY_MIME_TYPE()
+    Base.Multimedia.istextmime(::BINARY_MIME_TYPE) = false
+    Base.show(io, ::BINARY_MIME_TYPE, x::FriendlyData) = write(io, "Hello, $(x.name)!")
+    IJulia.register_mime(BINARY_MIME)
+
+    JSON_MIME_TYPE = MIME"application/vnd.ijulia.friendly-json"
+    JSON_MIME = JSON_MIME_TYPE()
+    Base.Multimedia.istextmime(::JSON_MIME_TYPE) = true
+    Base.show(io, ::JSON_MIME_TYPE, x::FriendlyData) = write(io, JSON.json(Dict("name" => x.name)))
+    IJulia.register_jsonmime(JSON_MIME)
+
+    FRIENDLY_MIME_TYPE_1 = MIME"application/vnd.ijulia.friendly-text-1"
+    FRIENDLY_MIME_TYPE_2 = MIME"application/vnd.ijulia.friendly-text-2"
+    FRIENDLY_MIME_1 = FRIENDLY_MIME_TYPE_1()
+    FRIENDLY_MIME_2 = FRIENDLY_MIME_TYPE_2()
+    FRIENDLY_MIME_TYPE_UNION = Union{FRIENDLY_MIME_TYPE_1, FRIENDLY_MIME_TYPE_2}
+    Base.Multimedia.istextmime(::FRIENDLY_MIME_TYPE_UNION) = true
+    Base.show(io, ::FRIENDLY_MIME_TYPE_UNION, x::FriendlyData) = write(io, "Hello, $(x.name)!")
+    IJulia.register_mime([FRIENDLY_MIME_1, FRIENDLY_MIME_2])
+
+    # We stringify then re-parse the dict so that JSONText's are parsed as
+    # actual JSON objects and we can index into them.
+    data = JSON.parse(JSON.json(IJulia.display_dict(friend)))
+    @test data[string(FRIENDLY_MIME)] == "Hello, world!"
+    @test data[string(BINARY_MIME)] == base64encode("Hello, world!")
+    @test data[string(JSON_MIME)]["name"] == "world"
+    @test data[string(FRIENDLY_MIME_1)] == "Hello, world!"
+    @test !haskey(data, string(FRIENDLY_MIME_2))
+
+
+end